This invention relates generally to vibration monitoring devices and in particular to devices for measurement of mechanical vibration amplitudes, on the order of 0.001 inch, where the frequency of displacement is high, for example, in the range of 20,000 Hz, and where large electromagnetic fields in the surrounding area make the environment hostile to electronic devices.
Prior art vibration monitoring devices generally may be classified into two groups according to whether the vibration sensing probe physically contacts the vibrating surface. In those devices which do make physical contact with the vibrating surface, the vibration transducer, or a probe which influences in some manner the vibration transducer, is biased by some means, such as gravity or a spring, against the vibrating surface so as to track the actual surface vibrations. These devices are inherently limited to low frequency applications, with the tracking accuracy decreasing as the frequency of vibration increases. They are not suitable for high frequency applications such as the measurement of vibration amplitudes in a material during an ultrasonic fatigue test.
The second group of prior art devices are more suitable for higher frequency usage and comprise those in which the vibration-monitoring transducer does not contact the vibrating surface. One such known prior art device is a capacitance gage.
Capacitance is the ratio of the electric charge on one of two adjacent electrically conductive surfaces to the difference in electrical potential between the two surfaces. Capacitance is a constant for any given pair of conductors, being directly proportional to the area of the aforementioned charged surface and inversely proportional to the separation between the two surfaces.
A capacitance gage measures the separation between the gage sensor and a target material by determining the existing capacitance through the relationship between charge and potential difference described above and comparing the known geometry of the sensor. Since the surface area of the sensor is known, the distance between the sensor and the target may be calculated. For this technique to be effective, it is essential that the target material be held at earth potential so that the gage may accurately ascertain the difference in potential between the two surfaces. When placed approximately 0.010 inch from the vibrating surface, the capacitance gage is capable of detecting the amplitude of high frequency vibration with good accuracy.
Two disadvantages, however, prevent the capacitance gage from being ideal in all applications. First, because its accuracy is dependent upon the target material being at earth potential, any current flow in the target material, whether inherent or induced by an exterior magnetic field, disrupts the accuracy of the gage. Consequently, the capacitance gage is not suitable for use in environments comprising large electromagnetic fields such as those found in an electric power plant. Second, because the gage is mounted to a surface which is mechanically independent of the vibrating material, the gap between the gage and the vibrating surface changes as a result of the thermal expansion and contraction of the vibrating material.
Where the vibrating material expands, reducing the gap between the gage and the vibrating material, the gage eventually contacts the vibrating surface causing an electrical short-circuit which renders the gage inoperable. Where the vibrating material contracts, enlarging the gap between the gage and the vibrating material, the separation of the two eventually exceeds the range of linear response of the gage, causing inaccurate amplitude readings.
It would be advantageous, therefore, to provide a device capable of measuring the amplitude of high frequency vibration such that the device is insensitive to the electrical activity within and surrounding the vibrating material and such that the device adapts to the expansion and contraction of the vibrating material so as continually to provide an accurate output of the amplitude of vibration.
A second type of known prior art device capable of measuring the amplitude of high frequency vibration is a gage based upon the eddy current principle. In this technique, an alternating current of a given frequency is generated in a primary coil producing a changing magnetic field surrounding the primary coil. This magnetic field induces an alternating current of the same frequency in a secondary coil located near the primary coil.
With the introduction of an electrically conductive target material into the magnetic field, eddy current flows induced in the target material by the original magnetic field give rise to an opposing magnetic field. This interaction results in a reduction of the flow of current in the secondary coil, the current flow decreasing proportionally as the target material is pressed deeper into the original magnetic field. It is thus possible to determine the separation between a gage housing the primary and secondary coils and a target material.
Because the gage need only detect current of a given frequency in the secondary coil, other electrical activity may be screened out making this gage somewhat less sensitive to electrical activity associated with the target material. Its immunity to such electrical activities, however, is not complete. The disadvantages of expansion and contraction of the target material, resulting in the electrical shorting of the gage or operation in a region of nonlinear response, plague the eddy current gage as well as the capacitance gage. The eddy current gage has an additional disadvantage in that it must be recalibrated for each target material which is comprised of a different type of metal. It would therefore be advantageous that the device capable of measuring high frequency vibration amplitude have the additional feature of lack of dependence upon the type of material whose vibration amplitude is being measured.
Hence, there is a need for an improved vibration amplitude measuring device capable of measuring high frequency mechanical vibration as well as lower frequency vibration, capable of tracking thermal expansion and contraction of the vibrating material so as to maintain accurate measurement of the amplitude and capable of operating without diminished accuracy in an electrically active environment.